The germicidal properties of metals such as silver, zinc, niobium, tantalum, hafnium, zirconium, titanium, chromium, nickel, copper, platinum and gold are well documented. Of these metals, silver, in the form of ions or compounds, is probably the best known and most widely used anti-microbial metal. Elemental silver has some anti-microbial benefit, but is generally too unreactive for most anti-microbial applications. An oxidized form of silver is considered to be more active as an anti-microbial as indicated by the observation that painting and inking of silver oxides leads to a decrease in their reactivity and solubility.
Attempts have been made to improve the reactivity of silver through the use of silver oxides and combinations of silver with other materials using accepted methods of solution-based chemistry. U.S. Pat. No. 4,828,832 describes the use of metallic silver salt solutions such as aqueous silver nitrate in combination with an oxidizing agent, such as benzoyl peroxide, to treat skin infections.
U.S. Pat. No. 5,824,267 discloses imbedding the surface of a plastic article with silver metal particles and ceramic or base metal particles to impart antibacterial properties to the plastic article. The extremely fine silver metal particles are obtained by chemical deposition from an aqueous silver salt solution.
Although solution methods of generating silver particles are able to provide anti-microbially active silver, there is little control over the structure of the resulting silver particles, so that these methods are limited in their applications. Moreover, some ionic species, such as aqueous silver nitrate, are too reactive for most applications because of the potential for skin irritation and must therefore be carefully monitored and controlled. Another problem with solution-based chemistry is the development of stable combinations without generating harmful byproducts. Silver ions bound in solutions of pastes, paints, polymers or gels tend to have a short shelf life, in part because of the side reactions with various constituents that can occur in water-based solutions.
There is a distinct need for anti-microbial surfaces that are capable of generating a sustained release of anti-microbial metal ions. The ability of a surface to generate a sustained release of anti-microbial ions would be particularly useful in surgical and wound dressings and bandages, surgical sutures, catheters and other medical devices, implants, prosthetics, dental applications and tissue regeneration. Other devices that would also benefit from a sustained release of anti-microbial materials include medical tools and surfaces, restaurant surfaces, face masks, clothing, door knobs and other fixtures, swimming pools, hot tubs, drinking water filters, cooling systems, porous hydrophilic materials, humidifiers and air handling systems.
A method for generating a sustained release of metallic ions is described in U.S. Pat. No. 4,886,505. According to the method, a device is coated with a first metal, such as silver, and a second metal, such as platinum, which is connected to the first metal through a switch. The presence of the silver and platinum metals in the presence of body fluids results in a galvanic action which is intended to release or liberate silver ions. The release of ions is controlled by the switch, which is operated external to the device.
The technique of applying a current to a silver-coated wound dressing or medical device is also the subject matter of U.S. Pat. Nos. 4,219,125 and 4,411,648. Although the use of external switch controls or an external electric current can enhance the rate of metal ion release, such external controls or currents-may not be practical for a variety of applications.
U.S. Pat. No. 6,365,220 describes a process for producing an anti-microbial surface that provides a sustained a release of anti-microbial ions without the need for an external electric current to maintain the release. According to the disclosure, multiple layers of metallic thin films are deposited on a substrate using a sputtering or evaporation processes. By using different metal combinations for the different layers, and employing etching techniques to roughen or texture the surface of the layers, multiple microlayer interfaces can be generated. The multiple interfaces, when exposed to body fluids, provide for release of ions by galvanic and non-galvanic action.
U.S. Pat. No. 5,837,275 also discloses anti-microbial coatings that provide a sustained release of anti-microbial ions. Coatings are prepared by a sputter technique using specific deposition parameters. The coatings are described as metal films exhibiting “atomic disorder” which is claimed to be required for sustained release of metallic ions.
Single ordered crystals of tetrasilver tetroxide (Ag4O4) are claimed to be useful as an anti-microbial in treating skin diseases (U.S. Pat. No. 6,258,385.) Such a composition, however, is not practical for other than topical use, and its ability to provide a sustained release of anti-microbial materials over a long period of time (i.e. several days) without reapplication, has not been demonstrated.
Deposition of anti-microbial materials is commonly limited to one of three distinct methods for producing silver and silver oxide coatings. Each of these methods has serious disadvantages and none have been developed to efficiently produce highly adherent, evenly distributed anti-microbial films on surfaces of medical devices and instruments. Commonly used state of the art processes, such as sputtering, dip and Ion Beam Assisted Deposition, produce coatings with limited adhesion to flexible substrates or elastic devices. Additional layers to increase adhesion are sometimes necessary at a significant cost in processing time.
Deposition of metal materials on a substrate by cathodic arc in a vacuum is known in the art. In contrast to other plasma vapor deposition methods, ion plasma deposition (IPD) can produce dense multi-component coatings of high purity as described in U.S. Patent Application Pub. No. 2004/0185182. However conventional cathodic arc deposition methods suffer from certain disadvantages. A waste of expensive material can occur due to inefficient use of the target material and the lack of particle control. The lack of control over the material being deposited can result in the formation of particles of varying sizes, which leads to the deposition of non-uniform coatings. Typically the cathodic arc processes also require the substrate surface to be heated to very high temperatures, which can damage the substrate material and severely restrict the choice of substrates.